Transport rollers of this kind are used in glass cooling passages for sheet glass production.
In the so-called float process, the sheet of glass floating on the tin bath is lifted off by means of a roll. Transport or drawing rollers lead the glass ribbon through a cooling passage for further processing. In order to maintain a fixed width and thickness of the ribbon a fixed temperature profile is necessary when cooling the glass ribbon in the cooling passage. This also applies to other processes.
In the cooling passage zones in which a relatively uniform temperature prevails, rollers of metal can be used of which the alloy composition is suitable for the operating requirements regarding oxidation and corrosion. Ferritic, ferritic-austenitic or austenitic steels may be used; the tubular center part of the roller is generally produced by centrifugal casting. In order to guarantee an optimal quality of glass the surface of the roller has a high polish, e.g. a peak-to-valley height of Ra = 0.8 .mu.m. Accordingly, pores or slag inclusions are not permissible.
In the regions of the glass cooling passages in which the ambient temperature is non-uniform, for example where it is necessary to subject the glass ribbon to a strong blast of cooling air, rollers whose center part is rigidly fixed to the trunnions and whose surface is cylindrically smooth have not been found satisfactory. Temperature differences in the center part of the roller lead to curved deformations which increase with each rotation of the roller.
For this reason, rollers with their center parts coated with asbestos are used in the regions of the cooling passages having an uneven distribution of temperature.
Another possible way of avoiding deformation of the rollers consists in making the center part from disks arranged at a distance from one another. While this gives a better stability of the shape of the roller even with a very uneven temperature distribution, disk rollers have the disadvantage that the surface pressure is increased owing to the reduction in size of the contact surfaces, and as a result damage to the glass ribbon can occur which inevitably increases with the thickness of the glass.